In trucks that carry heavy loads, typically on semi-trailers, the primary suspension system, i.e., the suspension system between the axles and the frame, has been plagued by the problem of providing for heavy loads without excessive static deflection while isolating the frame and the cab from shock and vibration. Suspensions utilizing mechanical springs were unable to solve the problem completely, for springs of relatively high rate are needed in order to support the load without excessive static deflection, but such springs are not very effective at shock and vibration isolation.
The introduction of air spring suspensions with active leveling helped considerably, for with air springs (1) a fairly low spring rate can be used without worry of excessive static deflection; undue settling under load can be prevented by automatically increasing the pressure in the air springs until the desired ride height is reached, and (2) the spring rate is proportional to the internal pressure of the air spring, and this is proportional to the load supported, so that air spring suspension systems inherently have a constant natural frequency, regardless of load.
However, despite these improvements, the ride quality of typical trucks has left much to be desired. A feasible way of improving ride quality is to provide a secondary suspension system interposed between the frame and the cab. The present invention relates to improvements in this kind of system.
Heretofore, there have been systems supporting only the rear of the cab by a spring, but this approach has had limited effectiveness. Four-point suspension systems utilizing metal springs have been somewhat more effective, but it has been difficult to get a spring rate low enough for really good vibration isolation without encountering problems with excessive static deflection.
A four-point cab suspension using air springs is shown in two U.S. patents to Foster, U.S. Pat. Nos. 3,944,017 and 3,948,341, the latter patent applying to a tilt-cab truck. While this system provided an improved ride, the system has generated and failed to solve a variety of problems. These include:
1. The Foster system uses conventional automotive hydraulic shock absorbers with internal elastic jounce and rebound stops. The type of damping these shock absorbers provide is not appropriate for a secondary suspension system, and the use of elastic stops necessitates a very large amount of system travel, i.e., vertical movement of the cab relative to the frame, of several inches, which in turn leads to several difficulties, enumerated below.
2. The large vertical movement of the cab relative to the frame, as described above, has precluded the use of any of the usual simple means of actuating the clutch, and has made necessary the use of a hydraulic clutch actuation system. Such systems are notoriously expensive and trouble-prone.
3. The shift tower and subfloor arrangement used in Foster's invention has been inadequate to prevent the intrusion of objectionable noise and fumes into the cab. Furthermore, in Foster's system, the shift lever is affixed to the vehicle frame, which results in a large and objectionable amount of relative motion between the driver and the shift knob as the cab moves up and down on its suspension.
4. There has been excessive roll of the cab from side to side.
Each of these problems is briefly discussed in somewhat more detail: